Fuel Cut Defeat (or Defender or Defencer, but not really a word). It's part of a method used to raise your stock boost level by altering the signal going to your ECU. Fuel cut is what your car does to prevent overboost, which will cause your pistons to melt by leaning out the air-fuel mixture too much. The FCD tells your ECU that your boost is at a lower level than what it really is so that it can continue being fed fuel at a higher boost level. You also need a boost controller to prevent the wastegate from opening and bleeding off boost pressure. It is said that our cars can actually be safely boosted up to about 15 psi with the stock turbo, intercooler and fuel system. The reason the factory sets it lower (about 9-10 psi stock) is for better fuel economy and reliability.